1. Field of the Invention
An aspect of the present invention relates to a thin film transistor and a fabrication method thereof, and more particularly, to a thin film transistor and a fabrication method thereof in which one excimer laser annealing (ELA) makes a pixel portion and a driver portion different from each other in surface roughness and grain size of a semiconductor layer.
2. Description of the Related Art
Many kinds of display devices are used for displaying an image. Recently, various flat panel display devices have been developed as alternatives to a Braun tube (i.e., a cathode ray tube). Such flat panel display devices are divided into emissive and non-emissive display device types, according to light emission characteristics. For example, an emissive display device includes a plasma display panel (PDP), an organic light emitting display device (OLED), etc., and a non-emissive display device includes a liquid crystal display device (LCD), etc.
The LCD and OLED are divided into active and passive types according to whether a thin film transistor is needed as a driving device. In the active display device, an amorphous silicon layer and a polycrystalline silicon layer may be used in a semiconductor layer of the thin film transistor.
Particularly, a pixel portion and a driver portion of the active OLED are different from each other in a required characteristic of the thin film transistor. For instance, the required characteristic of the thin film transistor in the driver portion is high electron mobility, but the required characteristic of the thin film transistor in the pixel portion is focused on uniform brightness.
Conventionally, to make the thin film transistor have different characteristics, the pixel portion uses an amorphous silicon layer for the thin film transistor, but the driver portion employs a polycrystalline silicon layer which is formed by laser beam annealing or the like for the semiconductor layer of the thin film transistor. However, in this case, the electron mobility is so low that it is difficult to drive the pixel portion. To solve this problem, annealing such as solid phase crystallization (SPC), metal induced lateral crystallization (MILC), metal induced crystallization (MIC), metal induced crystallization using a cap layer (MICC), or the like is applied to the pixel portion, and laser annealing such as excimer layer annealing (ELA), sequential lateral solidification (SLS), or the like is applied to the driver portion. However, since the annealed polycrystalline silicon layer has a number of defects in a crystal grain, a lot of leakage current is generated and the crystal grain decreases in uniformity, making it difficult to display an image with uniform brightness. Further, not only does it take a lot of time to perform the annealing, but also an additional process such as the laser annealing causes a fabricating process to become complicated. Even though only the ELA is used to make a crystal grain size of the semiconductor layer different between the pixel portion and the driver portion, both the pixel portion and the driver portion are fully scanned once, and then only the driver portion is scanned once again. Therefore, the time taken to move a stage is lengthened, thereby increasing a processing time.